1. Field of the Invention
The present invention relates to a doubly-fed induction generator, and in particular, to a controller of a doubly-fed induction generator.
2. Description of the Related Art
A doubly-fed induction generator is a representative type of a generator which shares 50% of a wind power generation system market, and is also researched and developed for tidal current power generation, wave power generation, and hydropower generation. The market for the wind power generation system is rapidly growing enough to exceed 30% annually, and is expected to grow toward the large-scale system in the future. In addition, researches, leading to commercial use, on tidal current power generation, wave power generation, or hydropower generation for developing dispersed power generation facilities are reported.
Among dispersed power generation facilities, the wind power generation market has the largest share, and 50% of the wind power generation markets employ the doubly-fed induction generator. Accordingly, not only a new design approach on the doubly-fed induction generator itself is getting attention but an approach of controlling the doubly-fed induction generator is significantly getting attention.
In particular, the tendency of the doubly-fed induction generator is toward the large-scale type more than 3 MW while developments on the doubly-fed induction generator are toward lowering unit cost of generation [VkW] consumed to generate a unit power, which inevitably leads to a design for the generator voltage not less than 600V. Accordingly, the conventional approach of low voltage driving has a limit to control the doubly-fed induction generator.
FIG. 1 is a diagram illustrating a topology about a controller of a doubly-fed induction generator disclosed in U.S. Pat. No. 6,856,040. The controller uses a silicon-controlled rectifier (SCR) between a stator coil and a grid side coil of the generator to synchronize the grid voltage. However, this approach uses compulsive coupling in terms of hardware while voltage vectors are not synchronized with each other so that an inrush current may occur in the grid coupling step and it is difficult to expect a rapid response. This approach also uses a capacitor for controlling a power factor in the stator coil. The value of the capacitor is determined so as to have the power factor become one in a near-rated driving condition, so that the power factor may become lower than 0.9 when the wind speed condition is far from the near-rated specification. In addition, among currents flowing through the rotor coil, only the magnitude of the current applied to the rotor from the grid side can be controlled, so that a reverse current flowing toward a DC link side from the rotor coil can not be used as energy at a speed greater than the rated speed in terms of structure.
To summarize the disclosed technique to which the present invention is related, the approach disclosed in FIG. 1 (U.S. Pat. No. 6,856,040B2) is applied to more than 95% of the wind power generation systems to which the doubly-fed induction generation is applied, however, it has drawbacks that an energy generated from a rotor coil can not be regenerated toward a grid side and an active power and a power factor of the stator side of the doubly-fed induction generator can not be directly controlled. In addition, about 20% of the generated energy is regenerated toward means of the rotor coil, which is consumed as heat so that the energy availability is low, however it has a simple structure so that it requires a low cost.
FIG. 2 illustrates a topology used for a regeneration type inverter in a motor driving manner, which discloses a doubly-fed induction generator capable of bi-directionally controlling an energy flow by applying a 3-leg insulated-gate bipolar transistor (IGBT) to both ends of a DC link (U.S. Pat. No. 5,798,631A, WO 2004/098261 A2). It does not additionally use a switching element or a capacitor for controlling grid side voltage synchronization and power factor and ensures a complete control of one for the power factor regardless of a wind speed. However, it disadvantageously has a high voltage distortion factor and is not perfect in terms of grid voltage synchronization, and is required to have an improved technique which can ensure high quality of maintenance and safety of the power grid when the grid is electrically interrupted or shortly interrupted.
FIG. 2 illustrates a technique which improves technical drawbacks of FIG. 1, which can thus control a power factor and a power to be regenerated toward the stator side of the doubly-fed induction generator regardless of the magnitude of speed and load and can regenerate 20% of energy regenerated toward the rotor side of the doubly-fed induction generator in the grid side (U.S. Pat. No. 5,798,631A, WO 2004/098261 A2). However, this approach has a topology mainly used for controlling a generator having a low voltage specification because it is based on a 3-leg IGBT module, and has a structural drawback that it corresponds to a 2-level converter having only two kinds of line voltage potential of an output voltage so that a distortion factor is very high enough to exceed 10% in a low speed region far from a rated speed. Such a bad voltage distortion factor becomes worse when a high voltage doubly-fed induction generator having a high voltage specification is driven. In order to drive the high voltage generator, the voltage also increases, which inevitably leads to a higher DC_link voltage, and a variation width of the voltage (dv/dt) varies up to two times the DC_link voltage, which increases more than the case of low voltage. When the generator is controlled by a signal having a significant variation width, the generator may be broken due to breakdown, and a reflective signal may occur all the time when the generator is driven by a control signal from a converter, so that an overlapped result may return to the input side when a voltage distortion factor is high, which limits the spaced distance, usually not more than 20 [m], between the converter and the generator.
FIG. 3 illustrates an H-bridge multi-level topology developed for driving a motor (U.S. Pat. No. 5,625,545A, U.S. Pat. No. 6,014,323A, U.S. Pat. No. 6,236,580B1). Several single-phase converters each composed of 2-leg IGBT can be stacked in a serial manner to generate a voltage waveform having a multi-level potential so that the high voltage distortion factor of FIG. 2 can be significantly decreased. However, a direct current voltage used for each single-phase converter must be isolated from each other so that a multi-stage transformer is additionally required for an input end. It is not reported yet that the voltage is used for controlling a doubly-fed induction generator so as to be used in a dispersed power generation facility.
FIG. 3 illustrates an H-bridge multi-level converter, which can generate a multi-level voltage waveform and an output voltage to allow for a high voltage specification so that it can control a motor such as an induction motor (U.S. Pat. No. 5,625,545A, U.S. Pat. No. 6,014,323A, U.S. Pat. No. 6,236,580B1). It employs a method, which applies a coordinate transformation manner allowing a motor to rotate in the same frequency as the rotating frequency, transforms various signals into a d-q rotating coordinate system, and includes various control strategies to restore the frequency same to the rotating frequency. When such a method is employed for controlling the doubly-fed induction generator, 0 [Hz] is generated, which results from the difference between a voltage frequency induced to the stator coil and a grid frequency, so that grid coupling can not be made, which leads to degraded doubly-fed induction generator that can not generate a power source from a dispersed power generation facility. Accordingly, a control algorithm capable of controlling the doubly-fed induction generator must be developed, which is not reported in the H-bridge multi-level converter type. Furthermore, a control algorithm is not yet reported which is developed for having a fault ride-through function, an anti-islanding function and a grid voltage synchronization function which are specifically required when a grid-coupled generator needs to be controlled.